The charge electrode for an ink jet nozzle is placed in the path of the stream at the point where the stream breaks into droplets. If the ink is conductive and electrically grounded, a voltage between the ink and the charge electrode will create an electrical charge in the stream adjacent the charge electrode. As the droplet breaks off from the stream, part of this charge is captured in the drop. The charged drop may then be controlled in trajectory by deflection plates placed along the path of the drop from the charge electrodes to the printing media.
For uniform charging results it is desirable to have a charge electrode that substantially surrounds the droplets with an electrical field during drop breakoff. Also, to increase speed it is desirable to print with multi-nozzle arrays, thus requiring companion charge electrode arrays. To increase resolution it is desirable for these charge electrodes to be placed on centers a few hundred microns apart. These constraints tend to create a fragile structure which is susceptible to damage by fracture or electro-erosion.
In the past, charge electrodes have been configured by cutting slots in substrates or drilling holes in substrates. The nonconductive substrates are then plated with a conductive layer to form the charge ring or charge slot. U.S. Pat. No. 3,975,741 issued to Eric R. Solyst gives a rather complete review of typical charge electrode structures of the past. Solyst further points out that such charge electrode arrays have been susceptible to damage by electro-erosion. Electro-erosion occurs because the ink is conductive and an ink mist inevitably contaminates the charge electrodes. Since adjacent charge electrodes may have different voltages applied thereto, the combination of conductive ink contamination of the electrodes and voltage between the electrodes causes electro-erosion of the electrodes.
In addition, the ink itself can be quite basic having a pH as high as ten. This alkaline solution can gradually corrode the electrodes. Thus, plated electrodes in the order of two or three microns thick can very quickly be destroyed by the ink or by electro-erosion between adjacent charge electrodes.
One attempt at preventing such electro-erosion and corrosion is to coat the plated conductive layer with an insulating layer of glass. This has the disadvantage that charge may collect on the insulating layer and partially inhibit the charging of the ink drops. In addition, the protective coating is subject to pin holes or other defects and, as a result, the ink seeps through to the conductive layer to corrode or electrically erode away the conductive layer.
Another attempt at solving the electro-erosion problem is taught in U.S. Pat. No. 4,035,812 issued to VanBreeman et al. VanBreeman et al teaches the placement of a resistor in the charge circuit path so that if the ink does short two electrodes, the resistor will limit the current flow and, hopefully, limit the damage to the charge electrodes. While this should help the electro-erosion problem, it creates a substantial fabrication problem in trying to place a bulk resistance near the charge electrode.